Barlowed Laser Collimation

[patomlin76]

Hi,

I'm after a bit of help regarding my attempt at laser collimation. As recommended to, I am combining my 2x Barlow with the laser collimator - however the two results are very different.

Whilst using the Barlow I have successfully centered the doughnut with my 45' viewing screen. Take the Barlow away though and the laser dot on its own is way off to the left.

Is this the possible margin of error not using a Barlow gives on a fast f/5 scope??

Hoping I'm nearly there!!!

Thanks in advance for any advice

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[Mr Spock]

It may be how the collimator is being held in the eyepiece holder. Have you tried rotating the collimator to see if the dot forms a circle? It may not be centred.

[patomlin76]

Ok then, I thought the secondary may have been throwing things off, so recollimated that and recentered it. Now the dot appears to be around 3mm off centre without the Barlow, so this seems closer?

I'll be fine tuning tonight in a darkened room to get the laser doughnut spot on. Does this seem a bit more realistic?

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[malc-c]

The general consensus is that Lasers need collimating first. It's no good trying to collimate a telescope if the laser beam is not actually central to its axis. There are lots of ways to check your colimator is colimated, but basically rolling it along a flat surface as seeing if the dot casts a circle is one way to hi-lite the issue.

[SniffTheGlove]

With my Baader Mark III the instructions inform you to align the collimator with the drawtube screw lock before collimating. I notice that if I rotate the colli in the draw tube then the laser does make a small circle.

[John]

You need to get the laser in the collimator adjusted so that it stays on the same spot when rotated through 360 degrees at at least 3 metres distance, preferably more. It's fiddly but well worth the trouble.

[patomlin76]

Right, tested laser and at first glance did not move when rotated through 360 - the previous owner had collimated it so it was never going to be far out.

Secondly on redoing secondary alignment, then barlowed laser, I find that the dot position between laser and Barlow laser shifts widely - see the photos attached.

If I rotate the laser in the drawtube it does not move when through the Barlow, tried with both 2x and 3x. Not sure which to trust!!

I've now got to test on a clear night to see what it looks like, but theoretically is it looking right or very wrong???

Thanks for the help, it's harder to explain these situations than to actually be here and see it! Hope it is making sense to you all.

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[earth titan]

Check the centre dot on the primary. These can be out by a long way and may account for the problem.

[patomlin76]

Cheers for the tip.

I had to change the batteries yesterday and now the actual laser needs collimating. I've tried and can't achieve the unmoving circle on a wall and really feel I made the wrong decision.

Lasers are for some people, just not me. It is now on eBay at a bargain price and will be replaced with a standard Cheshire sight tube.

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[umadog]

Just to clear things up a little: the laser does need to be collimated for the secondary adjustment because you're pointing the beam at the primary spot. It doesn't need to be collimated for the barlowed laser (read about it here: http://www.cameraconcepts.com/barlowed%20laser%20collimation.pdf). The reason is that you're not using the laser as a beam here. Instead, you're illuminating the donut and casting its reflection back onto a screen located in the focuser. Basically this is doing what a Cheshire does.

The reason this is a good idea is because the return beam of an unbarlowed laser is very sensitive to the collimation of the laser. The primary adjustment has very low error tolerance, so you really don't want the tool (the laser) to mislead you. Barlowing the laser removes the need for accurate collimation of the beam and so improves accuracy. You're just casting a shadow of the center spot back onto the screen at the focuser. You're no longer using the laser as a "thin beam of light."

Due to the above, you want to do the focuser rotation test *without the barlow*. It should be obvious from the above why this is the case (read the link too). If it's not obvious after that, then ask! Probably what's happening is that your laser is slightly miss-collimated so the return beam is going to the wrong place but the barlowed spot is correct.

Whilst you should check your center spot, I don't think that would explain what you're seeing. I believe a miss-placed centre spot should produce different symptoms. Namely your collimation tools (whatever they may be) would indicate that you're collimated (and agree with each other) but a star test will tell you otherwise.

[patomlin76]

Umadog, that is very helpful thanks. Indeed when I had just got the laser I knew the secondary was there or thereabouts - I actually got the barlowed laser part spot on but then was confused by the mis-collimated laser bit above...

So in fact I was, at one point, spot on! Should have left it there! Laser is now off eBay, determined to get this sorted again. Like last time I could always do the first part with my collimating cap, then finish off with the laser barlowed.

Cheers again

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[earth titan]

I have both a laser and a cheshire. The laser is fairly simply collimated by creating a simple set of V blocks using 4 nails (2 at either end knocked in at 90 degrees to each other on a plank of wood.

Use this for the secondary alignment then the barlowed laser for the primary.

[patomlin76]

Yes I saw a bit in the Internet where a few nails in wood was all that was needed. Making mine tonight!!!

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[umadog]

The collimating cap is a Cheshire. So it's a tool designed to measure primary tilt. The barlowed laser also measures primary tilt. So they're both for the same job. The collimating cap can, at a pinch, be used to round the secondary under the focuser. It will do a fairly poor job of secondary tilt. The first pass of the laser will do a much more accurate job for secondary tilt.

[angusb1]

Hi Paul,

Just found this thread. I'm sorry you're having such a nightmare with the laser collimator. This is the way I would try to do it (as recommended by Jason D)

http://stargazerslounge.com/beginners-help-advice/133925-laser-collimation.html#post1734326

If you can't get it sorted that way send it back and I'll give you your money back.

[patomlin76]

Angus,

Thanks for the kind offer but I'm sure it's just me not getting it right and being impatient. It almost seems cheating to take the scope to Green Witch to have it collimated there, as I'm putting so much effort in at home! I'm determined to get the collimator collimated so I can proceed with the scopes collimation!

As I mentioned above its doubly annoying as I've had it spot on. The laser was slightly mis-collimated which was why the barlowed shadow was centered when the dot was off centre.

If I'm still struggling in a weeks time I may take your offer but sure I'll have it sorted by then!

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[patomlin76]

Well I'd like to say thanks to everyone on this thread who has helped.

This evening I took the time to collimate the laser down to a couple off mm using a home made V out of some screws on mdf. Amazing how easy it was when the patience was there.

Next popped it in the scope to align the secondary. When the dot was centered on the primary I looked through the cap and amazingly the mirror was perfectly centered.

Now im just waiting for dark to finely collimate using the barlowed laser technique.

What a feeling it is to have conquered it! And a convert to the laser method too, as next time it is a 2 sec job in the dark to check both secondary and primary alignment.

Now I can start using those BSTs and TMBs (when it's clear!)

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